[RobZL1]

I like both the fuel line/check valve restriction and the pressure sensor theories.

I think you could diagnose which might be the most likely by looking at your LTFT's.

If they are extremely negative, then it's likely the pressure sensor.
If they are not extremely negative, then it's likely the fuel line/check valve.

Why do I say that?

If it's the pressure sensor it would be severely overfueled and you'd see it in extremely negative LTFT. In more detail, say it wants 800psi and it's being told it's at 400psi. It's then going to raise the pump cycle to the full 130degrees eventually to try to get to 800psi (though it may already be there or above in reality). In that case, it's also going to be pushing a lot more fuel than it thinks it is, and you'll be seeing maxed-out negative trims to try to maintain commanded lambda.

If you are not seeing extremely negative trims, it's probably the fuel line/check valve. It's not getting enough fuel, and it raises the pump cycle to try and increase the pressure to desired. Since there isn't enough fuel, it only gets part of the way there, and the pressure reading provided by the sensor is accurate the whole time. At this point, it's actually getting the fuel it thinks it is, and you won't see odd LTFT's. If anything, they may be quite positive.

Shoot holes in this, please, but that's how I'd approach it on my car.